The Inhibitor of Apoptosis Protein-Binding Domain of Smac Is Not Essential for Its Proapoptotic Activity

Smac/DIABLO, a recently identified inhibitor of apoptosis protein (IAP)-binding protein, is released from the mitochondria during apoptosis and reportedly potentiates apoptosis by relieving the inhibition of IAPs on caspases. We now describe the molecular characterization of Smac β, an alternatively spliced form of Smac, which lacks the mitochondrial-targeting sequence found in Smac and has a cortical distribution in both human embryonic kidney 293 and breast epithelial tumor MCF-7 cells. Smac β, which binds IAPs in vitro, does not bind IAPs in intact cells due to cellular processing and removal of its NH₂-terminal IAP-binding domain. Despite its inability to interact with IAPs in cells, processed Smac β is proapoptotic, as demonstrated by its ability to potentiate apoptosis induced by both death receptor and chemical stimuli. Furthermore, expression of a NH₂-terminally truncated Smac mutant (Δ75), which lacks the entire IAP-interacting domain, potentiates apoptosis to the same extent as Smac and Smac β. Our data support the hypothesis that the main proapoptotic function of Smac and Smac β is due to a mechanism other than IAP binding.